Compensated amplifier



Aug. 23, 1949.

o. H. SCHMITT 2,479,970

COMPENSATED AMPLIFIER Filed Feb. 8, 1944 SOURCE OF FILAMENT POWER SOURCE OF FILAMENT ,9

POWER INVENTOR OTTO H.- SGHM/T T Patented Aug. 23, 1949 COMPENSATEDIAMPLIF-IER Otto IL. Schmitt, Port Washington, N. Y.,. ass'ignor to thev United States of America .as represented by the Secretarytof the Navy Application February 8, 1944,.Seria-l No. 521 559 1 Claim.

This invention relates to an improved compensated amplifier, and more particularly to an 1amplifier especially adaptedior use with afilament or a current supply having relatively-Wide voltage fluctuations.

Amplifiers of the type. employing thermionic vacuum tubes ordinarilyrequire a sour-ce of :power to heat the filaments or heaters of the vacuum tubes. Fluctuations in the voltage of this-power source cause the electron emission from the oathodes of the vacuum tubes to vary over wide limits. This variationin the electron emission, if the frequency pass band of the amplifier is such as to, includethe frequency of change-in cathode emission due to filament-voltage fluctuations, will produce a spurious signalzin the output of the amplifier.

In amplifiers foruse in applicationswhererelatlvely complete freedomfrom spurious responses is required, and where it isnecessary to energize the filaments of the-vacuum tubes from a, source of current subject to wide'voltage fluctuations, it is essential that some arrangement be pr vided substantially to compensate for the deleterious effects of these fluctuations. In thepashsuch compensation means have comprised relatively complicated andexpensive networkswhich, while providing satisfactory compensation, in themselves introduced many potential sources of trouble.

It is an object .of thepresent invention, there- -fore, to provide a compensation arrangement of improved and simplifieddesign, which nevertheless is capableof substantially reducing the spurious responses which may result from fluctuations in the voltage of the filament-supply source. By theme of a single suitablychosen additionalvacuum. tube properly associated with the amplifier vacuum tube, for example, animprovement of the order of 30:1 may be secured withthe arrangement of the present invention.

In accordance with the presentinvention, there is provided, in an amplifier including a vacuum tube having acontrol electrode and a thermionic cathode variable impedance, means for minimizing spurious responses in the output of the am plifier due to variations inthe temperature of the cathode. This variable impedance means is utilized to develop a source of direct-current voltage which is made to varysubstantially in proportion to the variations in the electron-emission fromthe cathode, and connections forv applyin this direct-current voltage to the control electrode in such'a manner that the mean plate cur rent of the vacuum tube is held substantially constant inspite of the variations, in theielectron emission from its cathode.

While the improved compensation arrangement of the present invention is adaptable for use in any type of vacuum-tube amplifier, its use is especially advantageous in amplifiers designed to amplify direct=current and extremely low-frequency voltages. In such amplifiers, spurious responses, whatever their cause, are usually, considerably more serious than in the case of amplifiers for voltages of higher frequencies. E5:- perimentshave indicated thatsuch spurious responses arein large part dueto fluctuations in the emission ofelectrons from the cathode of the am- ;pliiyingvacuum tube, such variationsin emission being .due in turn tofiuctuations in voltage of the filament supply. By utilizing thearrangement of the present invention, therefore, a very marked improvement in theperformance of theamplifier may be realizedin those caseswhere the filaments are .subi ected tofluctuating voltage.

For a better understanding of the invention, to-

gether, with-other and. further objects thereof,

reference is .made to. the following. description taken in. connection with the accompanying drawing,,,and, its scope will be pointed out in the appended claim.

In theaccompanying drawing:

Fig. v1 is a..circuit diagram, partly in block vform, of a compensatedamplifier arranged in accordance with the present-invention; and

Fig..2.shows, partly inrblock form, a modification oftheamplifierof- Fig. .1, also in accordance with. the present invention.

Referringnow .to Fig. l of thedrawing, there is shown an amplifier, having input terminals I, 2, and output terminalss and 4, including amp1itying vacuum tube 5.. Resistor fi-isconnectsdbetween; input terminals I and 2. Input terminal I is connected to one terminal of potentiometer l, the sliderof which is connected throughbattery 8 to :grid- 910i vacuum tubet. Input terminal 2 is grounded,.as isxalso cathode 18 of vacuumtube 5. Plate of vacuum tube .5 is connected tooutput terminal 3 and also,.through resistor 12, tea source of positive-potential indicatedat 13+, Output terminal 4is alsoconnected to 3+.

Cathode -13 of vacuum tube-l4 is connected to input terminal. 4. Grid J5 and plate lii of =vacuumtube Mar-e connected together and to one terminal of potentiometer 1. Filaments or heatare H and 48,. respectively of vacuum tubes 5 and M, are connected to a source or power 5| 9.

Inoperation, vacuumtube- 5 operates in the usual .manneras an amplifieniin a desired region of its characteristic curve as determined-by the voltage ofbattery -8.. A-signal applied to input terminals -l and 2,:therefore, appears across output terminals3 and "4 substantially unchanged in wave --form.butof appreciably greater magmtude. Assuming .for the moment that the voltage of soureeAl-l remains constant at its normal value, the small residual ;current. which flows through vacuum tube l4 operating as a diode produces a potential drop across potentiometer I having the in placing vacuum tube cuit components in the low-potential side of the polarity indicated. A portion of this potential drop, depending upon the setting of the slider of potentiometer l, is applied togrid 9 of vacuum tube with such a polarity as to supplement the voltage of battery 8. In practice, the voltage of battery 8 is so chosen as to allow for this relatively small supplemental voltage, so that vacuum tube 5 operates in the desired region of its characteristic curve.

Now let it be assumed that the voltage of source [9 rises appreciably above its normal value.

1 or both of vacuum tubes 5 and I4 is replaced.

The higher voltage which is now applied to filament ll of vacuum tube 5 causes the temperature of cathode Hi to increase, with the result that the electron emission from this cathode also in-' creases. The effect of this is to cause a larger plate current to flow through resistor I2 than would flow were the cathode at its normal temperature. The potential drop appearing across resistor I2 would thus be greater than that normally corresponding with the input signal applied to terminals 1 and 2. Such an undesired spurious response, however, is greatly minimized by the action of vacuum tube i4. Any increase in the voltage of source [9 obviously increases the temperature of filament I8 and hence of cathode l3 of vacuum tube 14. This in turn causes a larger current to flow through vacuum tube l4, thereby producing a larger potential drop'across potentiometer I. When a portion of this increased voltage is applied to grid 9 of vacuum tube 5, the bias voltage on this grid is shifted in a negative direction, thereby changing the operating point of vacuum tube 5 and somewhat decreasing its mean plate current. The magnitude and direction of this shift in the operating point is such as substantially to compensate for the increase in electron emission from cathode 10 which was described above. The net result is that the potential drop across resistor 12 is substantially the same as In one successful embodiment of the invention, in accordance with either of Figs. 1 and 2, an improvement of approximately :1 in stability was realized compared with a similar amplifier without compensation.

In this embodiment vacuum tubes 5 and I4 each comprised one-half of a type 6SN7 tube; resistor 6 and potentiometer 1 each had a value of 1.0 megohm; resistor l2 had a value of 10,000

I ohms; battery 8 had a voltage of 6.0 volts; and the that obtained under the normal condition of operation at the rated filament voltage.

Consider now the operation of the system when the voltage of source 19 is considerably less than normal. In this case, the temperature-of filament l7 decreases, as does the temperature and hence the emission of .cathode'lfl, both of vacuum tube 5. The obvious result would be a decrease in the current flowing through vacuum tube 5 and hence in the potential drop across resistor I2. The simultaneous decrease in the temperature of filament 18 of vacuum tube l4, however, causes a decrease in the current flowing through this tube and hence a decrease in the voltage developed across potentiometer 1. This in turn shifts the region of operation of vacuum tube 5 on its characteristic curve in such a sense as to increase its mean plate current. As a result, the

signal voltage which is developed across resistor I2 is substantially the same as that which-would be developed under conditions of normal operation at the rated filament voltage.

The modification of Fig. 2 of the drawing is substantially the same in arrangement as that of Fig. 1, and identical in operation. In Fig. 1,

input terminal 2 is grounded, and cathode l3 of vacuum tube I4 operates at a potential above ground. In the arrangement of Fig. 2, on the other hand, cathodes I0 and I3 are both grounded, and both of input terminals l and 2 are above ground. This rearrangement thus consists merely l4 and its associated cirinput circuit of vacuum tube 5, rather than in source of positive potential B+ had a voltage of volts. Source [9 was a battery having a nominal voltage of 6.3 volts, and the actual voltage ranged from 4t0 10 volts.

While there has been described What is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

In a direct current amplifier, the combination comprising input and output terminals, an impedance unit connected in shunt across said input terminals, a first thermionic vacuum tube having an anode, control electrode, indirectly heated cathode and a filamentary heater therefor, said heater being connected to a source of direct current, a variable impedance network comprising a second thermionic vacuum tube having an anode, indirectly heated cathode and a filamentary heater therefor, said heater being connected also to said course of direct current and responsive to the same variations of potential as the filamentary heater of said first tube, a potentiometer comprising an adjustable tapped resistor connected directly across the cathode and anode of said second tube, means applied to the adjustable tap of said resistor and the cathode of said second tube for connecting said variable impedance network in series with said first tube cathode, control electrode and impedance unit, a load impedance connected to the anode of said first tube and means connecting said load impedance across said output terminals, whereby variations in the source of direct current will not deleteriously affect the current through said load impedance.

O'I'I'O H. SCHMITT.

REFERENCES CITED The following references file of this patent:

are of record in the 

